1. Field of the Invention
The present invention relates in general to computers, and more particularly to apparatus, method and computer program product embodiments managing data in a hierarchical storage server storing data blocks on primary and secondary storage devices, the primary storage devices being in an active mode and the secondary storage devices being in an active or passive mode.
2. Description of the Related Art
Hierarchical Storage Management HSM is a data storage technique which automatically moves data between a primary and a secondary storage tier. HSM is sometimes also referred to as tiered storage. In HSM systems, data files that are frequently used are stored on high-speed storage devices of the primary storage tier, such as hard disk drive arrays. They are more expensive per byte stored than slower devices of the secondary storage tier, such as optical discs and magnetic tape drives. The bulk of application data is stored on the slower low-cost secondary storage devices and copied to the faster high-cost disk drives when needed. In effect, HSM turns the fast disk drives into caches for the slower mass storage devices. The HSM system migrates data files from the primary disk drives to the secondary tape drives if they have not been used for a certain period of time, typically a few months. This data migration frees expensive disk space on the primary storage devices. If an application does reuse a file which is on a secondary storage device, it is automatically recalled, that is, moved back to the primary disk storage. Due to this transparent file recall capability, the file remains accessible from a client application although it has been physically migrated to the secondary storage. HSM is implemented, for example, in the Tivoli® Storage Manager. For details, see http://www-.ibm.com/software/tivoli/products/storage-mgr/.
HSM is advantageous for file systems that contain a large quantity of inactive files that are used only sporadically. In a typical HSM system, 90 percent of the file data can be stored off-site on secondary storage devices. Only a small quantity of spare storage space is needed on the primary storage devices to recall a small portion of the file data from the secondary storage devices when the client application requests access to this small portion.
For this reason, HSM is, however, not well suited for databases. Database systems store table and index data in large datasets or files that are accessed randomly. Even if the tables and indices are assigned to different datasets or files, the database system can infrequently access all data at periodic points in time, for example, at the end of a month or year. Since the prior art HSM systems only provide a small quantity of free space on the primary devices to store recalled data, this free space would not be sufficient to store a large quantity of the recalled data temporarily.